(HealthDay News) -- Needle-sharing among drug abusers may play as big a role as risky sexual behavior in the transmission of syphilis, a new study suggests.

American and Mexican researchers interviewed more than 900 female sex workers in the Mexican border towns of Tijuana and Ciudad Juarez, which are adjacent to San Diego and El Paso, Texas, respectively. The sex workers, who were also tested for HIV and sexually transmitted diseases (STDs), operate legally in the two Mexican towns, which are located on major drug trafficking routes.

The researchers found that female sex workers who didn't have HIV, but tested positive for active syphilis infection, were more likely than those without active syphilis infection to inject drugs, to use illegal drugs before or during sex in the past month, and to have U.S. clients who had higher rates of drug use, including the use of injection drugs.

"As more than two-thirds of these women have clients from the U.S., our data suggest that U.S. men seeking paid sex across the border in Mexico are at considerable risk of acquiring and transmitting syphilis and other STDs," study co-author Steffanie A. Strathdee, associate dean for Global Health Sciences and chief of the Division of Global Public Health at the University of California, San Diego, said in a news release.

Failure to control the spread of syphilis among injection drug users could also increase the spread of HIV, the virus that causes AIDS.

"Because syphilis is a co-factor for HIV, when an HIV epidemic occurs among drug users, high rates of syphilis could be a contributing factor that promotes the HIV epidemic," Strathdee said. "Men who have unprotected sex with female sex workers who have syphilitic sores are much more likely to acquire HIV because the two organisms exacerbate each other."

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